None of these are hard problems. They are just annoying, they always turn up at the worst moment, and nobody teaches you what to do. Here is the short version for ten of them.
Most of these happen at the same time in your life: you are applying for something, renewing something, or sending papers to an office. You have the document. You have the photo. The website simply will not take it, and it does not explain why.
This is the most common one, and it hides a trap: there are two completely different meanings of "too big". One is the file size in KB or MB. The other is the size in pixels. A form usually means the first, but not always, and the error message rarely says which.
If it is KB, you need to compress the photo, not shrink it. Use the Image Compressor, put in the number the form asked for, and let it work down to that size. If it is pixels, that is a different job — see the next point.
Government and university portals love exact pixel sizes. Your phone camera gives you something like 4000 x 3000, which is nowhere near it. Compressing will not help here, because compressing changes the weight of the file, not its dimensions.
What you need is the Image Resizer. Type the exact numbers the form gave you. One warning worth knowing: if the shape you ask for is different from the shape of your photo, something has to be cut or squashed. Crop it to roughly the right shape first, then resize.
A photo shop charges around ten to fifteen euros and gives you six pictures you did not need. For most online applications you do not need the shop at all — you need the correct size at the correct resolution, which is usually 35 x 45 mm at 300 DPI in Europe, or 2 x 2 inch in the United States.
The Passport Photo maker does the sizing and can lay several copies on a 10 x 15 print sheet, so a normal print shop can print them for about one euro. It cannot judge whether your face is positioned correctly for a biometric photo, so read the rules of the country you are applying to before you rely on it.
Your CV is one file, your certificates are three more, the cover letter is another. The portal has a single upload button. This is not you doing something wrong — most portals really are built that way.
Put them together with Merge PDF. Get the order right before you merge: cover letter, CV, then certificates in the order you listed them. If your documents are photos rather than PDFs, run them through JPG to PDF first, then merge.
Six phone photos joined into one PDF can easily come to twenty megabytes. Merging does not compress anything, it just stacks the pages.
The fix is to go one step back. Compress the photos before you turn them into a PDF, using the Image Compressor. Doing it in that order gives you a much smaller PDF than trying to squeeze it afterwards.
An office asks for page 3 of your contract, not all twenty. Sending everything is slower for them and gives away more of your business than you needed to.
Split PDF pulls out the pages you name and leaves the rest alone.
Phones guess which way up a photo is, and they guess wrong often, especially with documents on a table. A sideways scan looks careless to whoever opens it.
Rotate and Flip turns it the right way round. Do it before you make the PDF, because fixing it afterwards means redoing the PDF.
Screenshots come out as PNG. Many older upload forms only accept JPG. There is nothing wrong with your file, the form is just fussy.
Image Converter moves a picture between JPG, PNG and WebP. As a rule of thumb: photographs belong in JPG, screenshots and anything with sharp text or a transparent background belong in PNG.
If you send a copy of an ID or a certificate to a company, a light watermark saying what it was for makes it much harder to reuse somewhere else. Banks and landlords are used to seeing this.
Add Watermark puts your text across the image. Keep it readable but not so heavy that the document itself is hard to read.
Sometimes it goes the other way round: someone sends you a PDF and you need one page as an image, to put in a message or to print.
PDF to JPG saves the pages as picture files.
All of these jobs can be done on plenty of websites. The difference is what happens to your file while it is being done. On most sites you upload it, a server somewhere changes it, and it comes back to you. For that stretch of time your passport photo, your pay slip or your rental contract is sitting on a company's computer, and you are trusting a privacy policy you did not read.
The tools on this site work the other way round. Your browser opens the file straight from your own phone, does the work there, and saves the result back to you. Nothing is uploaded, nothing is stored, and there is no account. That is not a marketing line — it is the reason the tools still work when your connection is bad, and the reason there is no server here holding anyone's documents.
If you are preparing a set of documents, doing things in this order saves the most redoing:
Ten minutes, and you do not have to start again at eleven at night because the portal rejected the last file.