Generally speaking, when we say "YxZ pixels" we mean Y-pixel wide and Z-pixel high, that is, width comes first.
This convention is maintained also in some places of the ACP (for example, Emoticons and Icons settings), but not in Attachments/Images fields (max size and max link size), where the two fields refer to height and width in this order.
Neither the description makes the distinction clear.
Also the error message indicates width first and then height, so it looks like you can't post a 800x600px image if the maximum dimension is 800x600, but you can if it is 600x800.

I suggest to swap the two fields in those two lines.