If you’re using high-resolution images, you might have a lot more pixels than you need. The number of pixels, not the file size, is what may cause Bloom to run out of memory. To see what the DPI (dots per inch) is, hover the mouse over an image:
(If you can’t load an image into Bloom at all, recent versions will report the size of the image when telling you what went wrong.)
You only need 300 DPI for photographs. You can use something like FastStone Resizer to reduce all your images to the DPI you really need. If your images are a maximum of 5 inches across, for example, you can tell FastStone Resizer to resize all the images in the book to a width of 5 * 300dpi=1500 pixels.
(Older versions of Bloom may report that they ran out of memory loading an image when the real problem is that the image file is messed up somehow. You could try opening the image in other programs or try to find a good version of the same image.)