"large" means hundreds of thousands or millions of records.
I doubt seriously that you have that many images in use.
I would feel comfortable using RAND() with up to a few thousand records, though if your tables are "wide" (that is, if there are many many fields in each row of the table) there are some tricks you might want to use to improve the efficiency of your queries.
An optimist sees the glass as half full.
A pessimist sees the glass as half empty.
A realist drinks it no matter how much there is.