If you took a screenshot, the image will be in etmain/screenshots.

Run your image editor, I'm using Paint Shop Pro.

Open the required image file.  A screenshot is likely to be too dark; if so, increase the brightness and the contrast until the image is good.

Then crop to focus on the relevant bit, and resize/crop again to 341*256.  Then resize to 256*256 - you will need to turn off the usual feature that aspect ratios be maintained during a resizing.

If you want the image to be black and white, convert it to greyscale.  Likewise, if you want to put text onto it, create new layers for the text.

Save the image as a PSP type (or the file type native to your editor) or you will lose the layer information, making later rework hard or impossible.

If you want the image to look like a crooked old photograph, like the original ET map images, resize to 232*232 and enlarge the canvas (not resize) to give you some whitespace around the image.  Select the 232*232 image in the centre and rotate 1 degree to the left.

Use Pakscape (the link is in lesson 10) to open etmain/levelshots/battery.tga with your image editor, then copy the 232*232 rotated image over the top of it - it should fit nicely within the white photo borders, and the surrounding black area is already marked out as a transparent area using the alpha channel.  In other words, you don't have to worry about creating a transparency mask, as it is already present in the battery.tga.

Now save the image as etmain/levelshots/tutorial.tga.

Run ET and admire your photo as the map loads...