Most figure rejections are a units mistake, not a quality problem. A journal specifies width in millimetres and resolution in dpi; your image editor shows pixels. Converting is one multiplication: millimetres divided by 25.4 gives inches, times 300 gives pixels. Frontiers asks for one column at 85 mm or two at 180 mm, both at 300 dpi at final size — 1004 px and 2126 px respectively. PLOS publishes the same idea already converted: minimum 789 px and maximum 2250 px wide at 300 dpi, with 2625 px maximum height. Nature caps supply at 300 ppi. Get the pixel width right first and the file size usually follows, because a 4000 px wide figure compressed to fit a 1004 px slot is four times the pixels the journal will ever use. Resize down to the slot, then compress to the byte cap — in that order, never the reverse. Skwish resizes and compresses in the browser with a per-file before/after readout, so you can confirm each figure landed on the intended pixel width before exporting. Nothing is uploaded.