Pictures exist in various file formats (file types), such as JPEG (.jpg), PNG (.png), TIFF (.tif), or Bitmap (.bmp).
Consider the following:
If you choose a *.bmp picture, Bloom will convert that picture to *.jpg. This reduces the size of the file.
*.jpg format is only suitable for photographs, but not for line art.
The pictures from Art of Reading are typically *.png.
Scanned pictures usually become *.tiff or *.png files. If you have a choice, use *.png.
If you convert a line art picture to *.jpg, it will cause errors that you can see as a noticeable reduction in quality.
If you convert that picture again to another format, those errors will follow into the new format. This is true even if the new format is the format that was originally used.
Picture conversion happens if you use Save As and choose a different format for the picture file.
Hold your mouse pointer over a picture in Bloom to see a pop-up box. This box contains the information about that picture.
https://litmus.com/blog/png-gif-or-jpeg-which-ones-should-you-use-in-email