Why signatures are tricky Many application portals ask for a signature image between 10 KB and 20 KB. If you photograph your signature with a phone, the file can be 3 MB or more, and it may be a HEIC file the portal will not accept. Shrinking it too far makes the strokes blurry; not far enough and the upload fails.
Get a clean signature photo first Sign with a dark blue or black pen on plain white paper. Photograph it in good, even light without shadows, holding the phone straight above the page. Crop tightly around the signature — this matters most, because empty paper still costs bytes.
Compress to 10–20 KB This page is set to range mode with a 10 KB minimum and 20 KB maximum. Drop your photo above and press Convert. FitKB re-encodes it as a JPEG, searching for the best quality that fits, and adjusts the dimensions if needed. The maximum is met with a margin (20 KB means at most 19,600 bytes), and the minimum is counted as 10 × 1,024 bytes so the file is accepted either way the portal measures.
Check before uploading Use the 1:1 zoom checkbox to make sure the strokes are crisp. If the portal also requires specific pixel dimensions (for example 140 × 60), crop to that shape before compressing.
Privacy A signature can be misused, which is a strong reason to avoid upload-based converters. Privacy, in plain terms: FitKB runs entirely in your browser tab. Your photo is decoded and re-encoded on your own device using the browser canvas. Nothing is uploaded, there are no analytics or tracking scripts, and the page's Content-Security-Policy blocks connections to other servers. You can switch off Wi-Fi after the page loads and it keeps working.
Two exports per browser are free; FitKB Unlimited ($3.99 one-time) adds batch, presets and an offline version.