If you've got a local Stable Diffusion setup and a GPU that can handle it, sure.

Some other folks might be able to give recommendations for sites.

I'd try doing upscaling using the upscaler models, probably SwinIR_4x, which I use for general-purpose upscaling.

I'd do so using the SD Ultimate Upscale extension, which can do the upscaling in tiles so that it doesn't use VRAM proportional to the size of the image you want to upscale.

I don't know what you have installed. I tend to use ComfyUI, which is more powerful, but I'm pretty sure more people have the older and somewhat-simpler Automatic1111 installed.

Once you've got Automatic1111 up and running:

Automatic1111

SD Ultimate Upscale installation

If you don't have the SD Ultimate Upscale extension installed:

• Go to the Extensions tab.

• Click the "available" tab.

• Click the "Load from" button.

• Install "ultimate-upscale-for-automatic1111"

• Restart Automatic1111.

Upscaling

• Click the img2img tab.

• Either drag the image you want to upscale to the "Drag image here" square or click on it and select the image.

• Click the little yellow right triangle next to the "Resize to" area, which will copy your image dimensions to the image size fields.

• Set "Denoising strength" from what I think is 0.75 by default to 0.16. The 0.16 is a bit arbitrary, but it's roughly what you want when upscaling an image rather than regenerating it; 0.75 is too high. Feel free to try playing with it if you want.

• Go down to the "Script" drop-down menu and choose "Ultimate SD upscale"

• Check the "SwinIR_4x" upscaler. You can play with others, but this is one that should be reasonable for photographs.

• Change "Target size type" to "Scale from image size". Set "Scale" to "4".

• Click "Generate".

• Eventually your image will show up in the right hand square.

ComfyUI

ComfyUI works pretty much the same way: I'd also use the SwinIR_4x upscaler with the SD Ultimate Upscale node there. Can walk you through if you want, but at a high level:

• I'd install the ComfyUI Extension Manager. ComfyUI doesn't have the built-in ability to automatically download nodes, so you gotta manually install this one. You can also manually install and update nodes, but I'd recommend having the manager to make updates easier.

• Use the ComfyUI Extension Manager to install the SD Ultimate Upscale node.

• Set up a workflow that loads the image and runs it through the SD Ultimate Upscale node and then saves the image.

I haven't set up batch image processing of pre-existing images with either Automatic1111 or ComfyUI; my only batch-processing has been on generated prompts. However, I believe that that's also possible, so you can just feed it a large number of images and then let it run until completion. I couldn't off-the-cuff give directions to do batch upscaling, though.