I still am not sure if it's possible to batch import pictures, as I posed in the OP. If anyone knows if that's possible, let me know. I assume the mechanisms would double for attaching any files, too, so if anyone has experience doing that, let me know.
I actually figured my application would necessarily need a functional screen to update item pictures one-by-one as well. So I've set up that screen, and if there is no batching ability, a user will have to go around an update the item pictures one-by-one using this screen initially.
For that, I will share my solution. There is a filterable gallery - you must specify your facility in a dropdown, and then there are tabs for the different report types. The different report types have different items. You can then select items in that gallery. Once an item is selected, the default "Add picture" from the Media dropdown was used.
The Save button has the following code in On Select:
Patch(All_Items_Lists,LookUp(All_Items_Lists,ItemID=varItemSelected_PIC_UPDATER),{Item_Photo:UploadedImage1.Image});
Notify("Item picture updated successfully.",NotificationType.Success,1000);
Reset(AddMediaButton1);
The Patch statement calls the original data table hosting item data and where I want to send the picture. The LookUp calls the specific gallery item selected (selecting a gallery item sets the variable varItemSelected_PIC_UPDATER to the corresponding ItemID. Then we just choose the column value to update, Item_Photo, and patch in the UploadedImage1 from the default "Add picture" group in the screen. An important part of this function is making sure you have the ".Image" specifying your datatype. I assumed PowerApps sets the UploadedImage1 datatype to Image automatically, but that is not true. The function will not get flagged by the error handler for trying to patch a non-image datatype to an Image datatype in the dataverse tables - so that took me a bit to figure out...
The notify statement is self-explanatory. The reset statement will reset your image to the default on submittal, this way we're not accidently carrying the image forward if a user is saving additional photos for items and miss-clicks.
