A photo and a signature, the size the form asks.

Write down what the form asks.

Every form has its own rules, and they change, so take them from the form's own instructions, not from a list elsewhere. Note four things for each picture: the width and height (in pixels or centimetres), the smallest and largest size in KB, and the file type, which is usually JPG.

The numbers in this guide, such as 200 by 230 pixels and 20 to 50 KB, are examples of the kind of thing forms ask, not the rule for any one of them.

The photo: exact pixels and a limit in one go.

Open the image compressor and add the photo. Under More options, open Crop and rotate if you want to choose the framing, then Size: choose a custom width and height in pixels, unlock the proportions and choose fill and crop, so the photo fills the frame and is not squashed.

Under File type choose JPEG. Then turn on Must be under a size?, choose Other and type the largest size the form allows, such as 50 KB. Turn off Allow smaller dimensions under More options so the pixels stay exactly as the form asks; if the limit cannot be met at that size, the page says so.

The result shows the new size in pixels and in KB. Download it and check both against your notes.

• Open Photo and signature resizer
• Open the image compressor
• Guide: compress an image to 50 KB

Size: 200 × 230 px, fill and crop
File type: JPEG
Must be under a size: 50 KB
Allow smaller dimensions: off

When the form gives centimetres.

A size in centimetres needs a print resolution to become pixels. The image resizer does the sum: choose centimetres or millimetres, type the size and pick 300 DPI. A photo of 3.5 by 4.5 cm at 300 DPI is 413 by 531 pixels.

Then make that pixel size in the compressor as above, with your KB limit. The resizer also writes the print resolution into JPEG files, which the compressor does not; few forms check it, but if yours does, finish in the resizer and choose Small file under its quality.

• Open the image resizer
• Guide: a photo at passport size

The signature: lift the ink first.

Sign on plain white paper with a dark pen, and take a photo straight on in even light. A phone photo of paper is rarely white: it comes out grey or cream, with shadows, which looks muddy once it is small.

Open the Signature and logo background remover and drop the photo. It keeps what is darker than the paper and takes the paper away, and you download a PNG cut down to the signature. No model is downloaded for this; it works by brightness.

Now open that PNG in the image compressor, set the width and height the form asks for, choose JPEG and set the KB limit. JPEG cannot be see-through, so the clear parts become white: a clean white, not the grey of the paper.

• Open the Signature and logo background remover
• Guide: a signature without the paper

When the form asks for at least so many KB.

Some forms set a range, such as 10 to 20 KB, and refuse a file that is too small. The compressor keeps files under a limit and never adds empty bytes to reach a minimum. If your file comes out under the range, choose Printing or keeping under What is it for?, which keeps more detail and makes a larger file, or use more pixels if the form allows.

KeepSimp counts a KB as 1,024 bytes. If a form counts 1,000, a file right at the limit can be refused; aim a few KB inside the range.

Check before you upload.

Open each file once more: is the face clear, is the signature readable, is it the right way up? Look at the file's size in your file manager as well, since that is the number the form reads.

Everything here happens in your browser. The only upload is the one you make to the form.