The meme caption is one of the internet's few genuinely stable design conventions. Heavy uppercase white letters with a thick black outline, centred at the top and bottom of the image. It has looked like that since the mid-2000s and it has not needed to change, because it solves its problem completely.
Why that specific style
The outline is the whole trick. White text is unreadable over a light background and black text is unreadable over a dark one, and a photograph contains both, often within the same line of text. An outline in the opposite colour guarantees contrast against whatever is underneath, so the caption stays legible over a sky, a shadow, or a face without anyone having to think about it.
Uppercase does a similar job for a different reason: it has a uniform height and no descenders, so a line of it sits in a predictable band that is easy to place and easy to read at thumbnail size — which is how most memes are actually seen.
Filum uses the classic treatment: uppercase white letters with a heavy black outline, centred top and bottom, auto-sized to your image and wrapping onto more lines when the text is long, so it always fits without clipping.
What captioning does to the file
Drawing text changes pixels, so a JPG, WebP, or AVIF has to be re-encoded — there is no lossless way to add content to those formats. Filum re-encodes at high quality and shows the measured quality score of the delivered file, so the difference is a number rather than a shrug. A PNG stays pixel-for-pixel identical everywhere the captions do not cover, with transparency kept.
Your format is preserved either way: a JPG comes back a JPG, a PNG a PNG. Captioning marks the image; it should not convert it.
The part nobody thinks about
Memes are usually made from personal photographs and screenshots — a picture of a friend, a group chat, a work message, a moment from someone's camera roll. It is casual content, made quickly, which is exactly why the upload rarely gets a second thought.
It is still an upload. The photo of your friend, or the screenshot of that conversation, goes to a server belonging to whoever runs the meme site, before you have decided whether the joke works.
Filum stamps the captions inside your browser on your own device. The image never leaves your computer and never reaches a server. If the meme does not land, nobody else has a copy of it.