Slashing out the main panels and re inserting with graphics is easy and structurally sound, IF you stay away from the 'cross, mid-cord, etc. seams' that divide the spinn top and foot in the middle although if you do need to apply in that area then you must reinforce the main horizontal seam with an extra wide triple stitching, etc. A spinnaker concentrates almost all its stresses in the 'corners' so just stay away from the 'corners' when designing 'graphics'.
Painting is just as easy .... just do it BEFORE the panels go together. An 'insert graphic' wont eventually flake off as when 'paint' ages, and cause 'shape and flexibility' problems in the spinnaker cloth and will have the essentially same life-span of the other spinnaker cloth.
A spinnaker is a VERY easy sail to make DIY from scratch (especially with loft pre-cut panels, etc.) and that allows free reign for the application of graphics, even 'slash and fill' graphics.
The attached pic shows a graphic applied (slashed and cut then inserted) spinn.
The loft laser-cut the panels. I projected the image onto the side of a barn with the unsewn panels individually 'affixed' to the barn and then 'hand transferred' the image - panel by panel - and then overlaid the graphics material (same as the base ripstop and in the exact same fabric weft/warp orientation of the panel (important if you dont want 'stress puckers' or 'girts'), projected the panels over the projected graphic and outlined the panels ... and then added seaming allowances.
Each *individual* panel was then cut to accept the 'graphic inserts', the graphic 'insert' was added, and sewn to the base panel. Seaming of the graphics to the panels was done with stitching over 'pre-glued' (PECO double sided spinnaker seam tape ... sometimes used in small spinnakers instead of stitching) - I "PECO tape" all seams before stitching on all my sailmaking. Careful sail panel sewing insured that all the graphics were in line with the 'next' panel to be sewn. The 'details' were simply black/yellow 'ripstop' that was sewn to the base panels as an 'applique' after the sail was assembled/sewn. There are NO wrinkles, mismatches, creases from panel to graphic misalignment, etc. Time for the graphic application and set up time - about ~25 hours of extra sail making.
BTW ... that 'eagle' is approx 24+ft 'across'. and is 'glowingly translucent' in the sunlight because its 0.75oz. 'ripstop'. The sail is 170%LP at approx. 1775 sq. ft.