Using a Raspberry Pi is very wasteful in the first place.

If it's powered from USB, it doesn't really matter, but if you want to power it from batteries (especially something like AAA NiMH batteries) so that you can easily hang it on a wall somewhere and make it last a month or so (refresh every 10-15 minutes maybe), you pretty much have to use something much more low power (like an ESP8266, ESP32, etc.).

Having pre-stored pictures is also unnecessary, just generate them on the fly. Unless generating the picture takes a long time (e.g. having to contact a lot of slow external services), you can just draw it on the server in memory with each request.

In the same request, the device can also send its battery voltage to the server, where you can collect it as metrics.