SD cards go to 2 GB capacity and the SDHC (High Capacity) cards to 32 Gb.
Divide the 1GB (1024) my the size of the photo file in MB.....there for 1MB files will give you 1000 pics per GB or just less.....so ye the same reckoning ...half a MB will get you 2000 per GB.....
There's no such thing as a 'standard' photo. Even the cheap point and shoot digital cameras are around 10 megapixel resulting in each photo being around 3 mb each which would result in about 300 per 1gb memory card.
Just further to my post.....my Lumix 12 Mp camera gives 5 different standards of quality pictures from 5.2 MB per pic to just a mere 177 Kb and various in between sizes......a 4GB card gives me 783 best quality pics....and at the lowest an amazing 17,694.............what would be acceptable for a digital photo frame I wouldn't know.....I'll leave you to do the maths.
You should be fine to use SDHC in your SD device as thye are backwards compatible.