Why the typography is what it is
The white block-capital text with a heavy black outline is not arbitrary. It descends from Impact, the typeface bundled with Windows and chosen by early image-macro tools because it was condensed enough to fit long lines, heavy enough to read at thumbnail size, and present on nearly every machine.
The black stroke solves a real problem: the text has to remain legible over an arbitrary photograph containing both bright and dark regions. White alone disappears over a light sky, black alone over a shadow. An outline guarantees an edge against either, which is the same reasoning behind subtitle rendering in video.
Legibility at small size drives the rest. Memes are consumed in feeds at a few hundred pixels wide, so the text must survive aggressive downscaling and re-compression as the image is re-shared. Thin typefaces, subtle colours and tight tracking all fail that test, which is why the convention has stayed remarkably stable.
Formats and where they break
Save as PNG if the image contains only text over flat colour, and as JPEG if it is photographic — but be aware of what happens next. Every platform re-encodes uploads, and text over photographs is exactly the content JPEG handles worst, since sharp glyph edges are high-frequency detail. The visible decay of a much-reshared meme is generation loss compounding across a chain of re-encodes.
Start from the highest-resolution source you can find rather than an already-degraded copy, and export at a generous quality, since your file will be re-compressed regardless. Uploading at roughly the platform's display width avoids one resampling step.
For animated formats, GIF is limited to 256 colours per frame and produces large files; most platforms now convert uploaded GIFs to video internally anyway. If the destination accepts video, use it — the GIF converter is for cases where GIF is specifically required.
Copyright, and being funny without being sued
Meme templates are usually stills from films, television or photographs, and those are copyrighted works. Non-commercial sharing is very rarely pursued, and transformative parody has real protection under fair use in the United States and fair dealing for parody in a number of other jurisdictions.
Commercial use is a different matter and is pursued. Using a recognisable film still or a professional photographer's image in advertising, on merchandise or in marketing material is straightforwardly infringing, and stock photography agencies in particular monitor and enforce. Several well-known meme photographs have generated licensing settlements.
Identifiable people add a separate consideration independent of copyright: personality and publicity rights in many jurisdictions restrict commercial use of someone's likeness without consent, and a meme that mocks a private individual can be defamatory regardless of who owns the photograph. For commercial work, use licensed stock or images you took yourself.
Getting the classic face on every machine
Impact ships with Windows and macOS but not with Linux, Android or most ChromeOS builds, so a meme tool that names only Impact renders in whatever the browser falls back to on roughly a third of devices. The fallback list this page used to carry — Impact, 'Arial Narrow Bold', system-ui, sans-serif — did nothing at all, because Arial Narrow Bold is not a font family: the family is Arial Narrow and Bold is a weight. Measured on a canvas at 100px, the string HAMBURGEFONSTIV came out at 766px in Impact, 838px in real Arial Narrow, and 1044px under that bogus name — which is to say it fell straight through to a wide system sans, the opposite of the condensed look the style depends on.
The page now loads Anton, the open substitute for Impact, alongside a corrected local fallback chain. Canvas text is measured at the moment it is drawn, so the first render has to wait for the webfont to arrive — otherwise the line-wrap points are computed against the fallback metrics and shift as soon as the real face lands.
Captions are positioned from real glyph metrics rather than a fraction of the em. actualBoundingBoxAscent and actualBoundingBoxDescent report the ink of the exact string being drawn, which is what lets a caption sit flush against the top of the frame without slicing the tops off its capitals, and what makes the selection outline hug the text instead of floating around it.