Great job with the rack, should start producing and selling them :O)
Those 800 x 600 jpg's looks fine here, for landscape width use anything from 800 x * to maybe 1600 x * (or even up 2500 x * if wanting to provide possibility for zooming into more details). For portrait use the first number as height.
Only restriction is the 1500 kb (1,5 mb) file size, to curb any attempt at posting the 9 x 6 meters large pics that comes out of phones and cameras these days :0)
Smaller pics = fast loading (many are on slow or metered internet), larger pics = more details.
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And renamed topic + moved to 'Frame'.