File size and pixels are different things
If you hit an attachment size limit, look at the number of bytes; if you hit a photo dimension rule, look at the number of pixels. Compression that lowers quality keeps the pixel dimensions and reduces the detail stored. Resizing reduces the number of pixels itself. Two 640×480 images can have different file sizes depending on their content and format.
Screenshots with small text can get blurry edges if you lower the quality too much. Decide the pixel dimensions you need first, then compare at the size the image will actually be viewed. There is no feature that hits an exact target file size from a quality value.
Choosing the output format when compressing
Select JPG, PNG or WEBP files and choose the output format and quality. Each file can be up to 25MB, and multiple images are saved as a ZIP. Quality applies to lossy formats such as JPG and WEBP. The same quality value does not apply lossy compression to PNG.
If the result in the same format as the original is larger, the original is saved unchanged. A 0% reduction can be normal. If you chose a different format, the file is saved in that format even if it gets larger, so check the result’s size.
Technical source: Format and quality options of the browser canvas · Checked 2026-10-11
Keeping a transparent background
If you need a transparent background, such as for a logo or a cut-out product photo, consider PNG or WEBP output. If you choose JPG, this tool fills the background with white before drawing the image, so transparent areas turn white.
Do not simply pick the smallest file; place it on the background where it will be used and compare the edges. An unsupported output format shows an error rather than being saved with the wrong extension. Color profiles and metadata are not guaranteed to stay exactly as in the original.
Calculating scale and maximum dimensions
At 50% scale, 640×480 becomes 320×240. If you set a maximum width of 200px and a maximum height of 100px, the image keeps its aspect ratio and shrinks to 133×100 so it fits within both limits. Rounding can cause an aspect-ratio difference of about one pixel.
Maximum dimensions do not mean the image is forced into 200 by 100. The scale factor is min(1, max width ÷ original width, max height ÷ original height). Setting a larger box does not enlarge the image. Enter 1–100% for scale, and positive whole numbers for maximum dimensions.
When a file does not shrink or will not open
A JPG that was already reduced, or a simple PNG, may not get much smaller when processed again. Files whose extension was merely renamed, damaged images and unsupported formats cannot be opened. Keep the original and check the text, edges and colors of the result.
To meet a pixel rule first, use image resizing; to change only the format, use image format conversion. Even when you exceed a processing limit or memory runs short, files are never automatically uploaded to an external server.
Keep the original and check the content, size and format of the result.