Nothing, but loading images depends more on your graphic card bandwidth as they are loaded as pixmaps and then must be converted and sent to your graphic hardware.
At worst you are overloading your 8 - 32MB VRAM (iBooks don't have more, right?) because BM does not support S3 texture compression. Images are always in full size in VRAM in OpenGL, because BlitzMax does not support S3 texture compression, which would lower the VRAM size of an image (which is width * height * bitdepth, not the filesize you actually have).
At worst you are overloading your 8 - 32MB VRAM (iBooks don't have more, right?) because BM does not support S3 texture compression. Images are always in full size in VRAM in OpenGL, because BlitzMax does not support S3 texture compression, which would lower the VRAM size of an image (which is width * height * bitdepth, not the filesize you actually have).