A decisive factor for a good photo is its resolution. Images for laser engraving should have a resolution of at least 300dpi as the output size. If necessary, you can increase the resolution in your graphics software by re-establishing the bitmap (example in CorelDraw under “Bitmaps” - “Resample”). Internet photos often have a resolution of 72 dpi. Even these can be processed using a laser machine, if the original image is large and scaled down to workpiece size. You should avoid enlarging small images, as this decreases their resolution (the image raster becomes too large and is not suitable for photo engraving).
A comparison: well suited versus ill-suited images for photo engraving:
The left image is suitable for laser engraving, as it shows a good contrast, it is well exposed and the elements in the image stand out.
The right image is less suited for laser engraving. The people stand out from the background, but the exposure is too weak, making the people look like they are in the shadows. This makes it difficult for details to be perceived, and the people would be engraved as a unitary surface.