On the SD Card, there are probably 2 sets of pictures, and you may need to look at it on a laptop or computer. The first is a thumbnail, probably a short name like Thm_4589,jpg and there is the corresponding photo 20181127_4589.jpg.
There are limitations to using the SD card interface, such as the pictures need to be in a compatible format (jpg, gif, png...) and have an 8 character name and a 3 character extension. You are seeing the thumbnails because the name is short enough to be compatible. The picture may have a longer name like I have above. These will not be seen since the names are too long.
If the picture is a RAW file, or some unsupported format, they also will not be seen as the iPad can't open them.
You may need to rename the picture files to have shorter names, and I would delete the thumbnails to get rid of the grainy pictures.
See if that helps. If not, reply back with some specifics like file name, format, where the file came from and brand of SD to Lightning reader you have.