Without knowing the picture, I can provide different ways:
1) If you do not want your photo to be too noticeable in the background, try blurring it (it may be a lot depending on the image!) → then you can increase the compression ratio by not seeing artifacts too much.
2) depending on your image, try reducing it to certain color ranges (which you use the maximum number of colors) → using this method you can end up using a .png or gif file, photoshop will show you which is better.
Last thought: Since this is a BACKGROUND image, try saving it in the “back” - this way you can try and modify (destroy) the image in such a way as to reduce colors! On the other hand, try using smaller images (800x600) and let css scale them, again - depending on your intentions.
I use a 1280x1024px bg image with a speed of about 55 kb.
hope this helps.
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