Picture format advice: BMP: Do not use these. These are uncompressed pictures and hella bloated. They'll take a long time to upload, and download.
PNGs: PNGs are great if you want to archive photos or show game screenshots. The compression is great for archiving. But when it comes to posting stuff, the filesizes will just be TOO BLOATED for most people to download properly. A well-compressed JPG image can be much smaller than a comparable PNG image.
On the other hand, greyscale PNGs are pretty decent in size and compression, if saved properly (this depends on the program used). Stuff scanned from manga volumes or black and white books can be saved like this for maximum quality, but the file size will still be pretty high.
JPG: The higher the quality, the larger the file. The lower the quality, the lower the file, but you'll see more artifacts in the image. Most of the scans I post here are set to 75% quality (or less). Image editing software usually has a preview feature which lets you adjust the quality and see the final product before saving the image.
They're pretty good for black and white / grayscale images too.
GIF: These can only have 256 colors, so they're not really very good for posting scans. Avoid using this.
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When it comes to webcomics, the authors usually upload heavily optimized pictures so you can just save them and repost them without issue most of the time.