That is exactly how I was trying to do it but they call the background the canvass. I had no trouble opening a new file and sizing and cokouring that. It was just that when I brought in the photo to place on top I couldnt size it or crop it.
You have to resize the actual image as a separate file before putting it onto the background. Otherwise, it will be almost impossible to resize.
Yes, I am actually trying to do that now.
It’s important that the pixels/inch for the actual image are the same as those of the background. I use 240, like this:
[ATTACH=JSON]{“data-align”:“none”,“data-size”:“medium”,“data-attachmentid”:2946}[/ATTACH]
ok I see. Let me try this again.
I have another question about the rectangular selection tool. How can I set it to the correct ratio that I am looking for in my lithophanes which seems to be about 100: 150 .
[ATTACH=JSON]{“data-align”:“none”,“data-size”:“medium”,“data-attachmentid”:2951}[/ATTACH]
Please note that the Crop tool is selected at the top left.
OK thank you very much.
At the moment I have a Mona Lisa the right size above my back ground( canvass in their terms) and they are both at the same resolution. My problem is that mona is tucked into the top left corner instead of being centered so I have a 12 mm border in grey to the right and the bottom. Clicking align didnt work. AHA! I used the move tool and got it centered by eye. Thats ok I shall make another lithophane. Thanks so much for your patience.
OK, I assume you have the background and image files both open. Click on the image file. Click on the Select menu item and click on All. Now, click on the Edit menu item, then click on Copy. Now, click on the background file. Click on Edit again, then click on Paste. The image file should paste onto the background already centered.
I shall try that next time, I am already printing Mona for my lamp. You have saved me a lot of frustration.
A small suggestion: try it soon with Mona as practice.
It is in the process of printing as I write this.
You should still be able to use GIMP to try it out, or is the computer so slow you’re worried about it affecting the print?
You have taught me what I need to know to make my lithophanes for my lantern. I am taking a break frtom Gimp at the moment because my damaged brain finds concentration very difficult. I have to back off and do something else so I am trying to learn how to make a decent pic of a lithophane and my lamp to show it to people.
I print from an sd card and my new old mac is plenty fast.
Sounds like a plan 
I look forward to seeing some results.
I dug out an old digital camera and charged it up and am trying to see what I can do with that. My web cams just see a ball of light with no detail.
I could show the lithophane against a weaker light as I have done before but I would like to show the lamp working as we see it.
I’m surprised. I would have thought the exposure on the Mac’s camera could be set low enough to see the lithos. Too bad. I hope the camera fairs better.
Ah, gotchya. Makes sense. I guess a dimmer on the lamp would be handy 