your allowed any invoice reference you like. The consecutive thing is an internal and external auditing thing that helps identify invoices. There is no actual legal requirement.
There are rules for what has to be displayed on a VAT invoice such that the necessary input and output VAT can be correctly claimed but this is entirely different.
In your situation i would personally choose, as suggested, to add a client reference such as ACME-001, ACME-002, or such like if you wish to differentiate these invoices.
As for image - it cuts both way. General consensus is often the larger appearence of the co. the better i.e. more stable supplier, longivity etc. However as pointed out it can cut the other away re the personal service. It depends on the individual customer and noone can second guess that for sure.