Actually it's not all that odd.
Cats like pictures and plastic bags among other things because they are often manufactured with animal products. Plastic bags often contain fish oil, and pictures contain a few different animal products that make them shiny or keep them looking good. Many household products also contain gelatin, which is long-boiled pig skin and such, and though it doesn't smell like animal to us it may to them.
As for the envelope glue, I don't know, but it is probably something similar.
It's certainly not cause for alarm, although I would keep the photos out of reach because there are other chemicals in there not good and also because cats have a tendency to start chewing on them.
The glue on envelopes is probably okay, as we know it's been approved for humans. I would doublecheck, but it's likely benign.