What do you consider cheap?
Linux tablets
The Juno Tab 3 from last year was $699 (no more stock, Juno Tab 4 has been announced) and the Librem 11 from Purism is $999 (temporarily out of stock).

Tablets/Laptops hybrids
A refurbished Microsoft Surface Pro 5 or 6 are well supported by using the kernel from here:
https://github.com/linux-surface/linux-surface/wiki/Supported-Devices-and-Features#feature-matrix
There are newer Surface Pro tablets that works too unless you need LTE/Camera.
In a similar vein you can look at refurbished Lenovo 13’’ 2-in-1 devices but they will be bulky compared to a normal tablet.

Wall installation alternative using a touchscreen monitor and a mini computer
Putting a minicomputer like Radxa X4 in the backside of a touchscreen monitor like the Lenovo ThinkCentre Tiny-in-One is cheaper than both but requires constant power from the wall and wouldn’t be classic tablet format. (Works if you want a static wall installation though, just buy the wall bracket with VEGA 100x100 that’s right for your plans)

edit:
On the other hand if all you wanna do is display photos then maybe a digital photo display that supports USB or SD Cards would be the easiest solution?
It would require moving the storage back and forth to add new photos but you wouldn’t even have to give it WiFi so if you keep it isolated it is nice from a privacy standpoint.