Why is the BMP so much larger than my JPG?
Because BMP stores every pixel as three raw bytes with no compression at all, while JPEG throws away detail the eye is bad at seeing and then entropy-codes what is left. A 2 MB JPG at 12 megapixels becomes roughly 36 MB as a BMP. That ratio is normal and not a sign anything went wrong.
Does converting to BMP improve the image quality?
No. It cannot. JPEG loss is destructive: once the 8x8 blocks have been quantised, the original pixels are gone. Decoding to BMP writes those already-damaged pixels out losslessly. You get a bigger file with identical visible quality.
Should I use PNG instead?
Almost always. PNG is lossless like BMP but uses DEFLATE compression, so it is typically five to ten times smaller, and it is read by everything. Choose BMP only when a specific piece of software, firmware, or a hardware pipeline refuses anything else.
What is 24-bit BMP exactly?
Eight bits each for blue, green, and red, stored in that order (BMP is BGR, not RGB), with no alpha channel. That gives 16.7 million colors, which is enough for any photograph. The 32-bit variant adds an alpha byte, but a JPG has no transparency to preserve anyway.
Why are BMP rows stored upside down?
Because the format was designed with a positive biHeight meaning bottom-up row order, a convention inherited from the way early Windows device-independent bitmaps were laid out. The encoder writes the last image row first, so any BMP reader shows your image the right way up.
Is my JPG uploaded to convert it?
No. The JPG is decoded onto a canvas in the page and the BMP byte stream is assembled in JavaScript on your device. Nothing is transmitted, which is worth knowing when the BMP is destined for a device pipeline holding sensitive imagery.