I needed to store a large quantities of images so took the following measurements:
| Format | Time | Size | Sample |
|---|---|---|---|
| bmp | 0.715670 | 1769526 |
 |
| gif | 4.184417 | 501931 |
 |
| jpg | 2.507811 | 22252 |
 |
| png | 10.909442 | 67295 |
 |
| ppm | 0.648540 | 1769488 | Browser does not support PPM |
| tiff | 1.011216 | 1769600 | Browser does not support TIFF |
Gif is the clear loser - it takes a long time to process but still looks terrible.
For space use jpeg, speed ppm.
Google’s new WebP format looks promising.