I got tired of my workflow being a few squiggly lines on a piece of scrap paper in my desk drawer, and as the subject of 'Gimp vs. Photoshop' came up (yet again) in a Discord chat, I figured that I should re-format it, and that I might as well share it here.
Before I dive into the Gimp part (because obviously I use Gimp, which is superior and also free!), there are a few principles that starts in SL itself, and different workflows depending on what I'm doing. The rule of thumb is always to do as much work as possible in SL, and as little editing as required to reach the desired effect; this means that the pose, the backdrop, the lighting, the framing, and (almost always) the DOF should all be spot on before I open Gimp - and there's no crime in taking a shot to look at it, decide to adjust something and then move back to re-take it until it's where you want it, whether it's changing the camera position or the lighting intensity or the DOF sliders.
On a good day, the only editing required is:
Blend neck seams.
But sometimes you'll want to do something more 'artsy' in the editing process, and occasionally you've got a shot which requires more fixing (a shot not taken in a studio, or in a shoot you wrapped before realizing there was an error). If any of this is required, then you'll want to complete this before you start adding filters and effects - and it's also the point at which you do the layering, if you're doing a layered shot.
Did an animation wherein your shoulder popped backwards? Someone got a leg going through a skirt, or an arm going through their hip or gut? I don't know what the 'proper' term is, if there is one aside from 'correcting' it; I like to call it 'reconstructive photo-surgery', because I'm a weirdo. This assumes that there's only one layer of the shot, or that even with multiple layers, there's an issue that needs correcting (if you're doing image layering, that may come first though). At this stage you'll primarily be using the paint brush, smudge tool and heal tool, to rebuild the missing or broken things.
So what do I mean when I talk about 'image layering'? It's when you want to wear a hair and a hat, but can't get the two to work together in-world without extensive clipping or having to deform the hat until it's more of a tent than a bonnet. In a studio setup, you can solve this by taking two shots - one with the hair, and one with the hat, then lay the hat layer on top of the other one and use alpha masks to just put the hat on. Or a coat over a shirt, or a necklace that clips with your shirt collar, or anything else really that collides with something else.
Once you've got the shot fixed up so that there are no seams or clipping, you're ready to begin adding any effects or filters that you may want to do.
Personally I've found myself using and making filters less and less, not only because of the 'raw shot puritans' raging on the forums, but because I've learned to set up my lighting and stuff in the viewer - looking back at photos I did even just one year back, yikes, the excessive use of Softglow (why did I ever--) and other filters making the contrasts much too hard and the colours appear posturized, ugh. But there still are some features that I use to some extent on occasion. Sharpening (unsharpen mask) can be a great way to pick out some detail, especially on portraits, to make eyelashes and pores pop, layering a desaturated copy of the image (or just playing with the slider, because Gimp now has a slider - yay!) to make the shot appear, well, less saturated (gloomier!), and of course film grain (i.e. HSV noise - 'cause the cool kids do it!). I've also done a few shots where I've tested out the whole anaglyph thing (again 'cause of the cool kids) and found that .. Nope, just no. I'll leave those shenanigans to the pop star wannabe bloggers - after all, how dark do you appear when it looks like you're at a disco doing LSD?
And finally there's the signature - easily done, just make a signature that you copy-paste into your pic. Well, I used to do that at least, but I've gone from an underlined, two-line black letter logo with wavy effects to just a single line, all minuscule fraktur, which I just quickly type in by hand, align to the corner I want it in, and then drag it out a few pixels from the edges and set to a high transparency to make it more discrete. But that's just me.
Glances up at the text wall to figure out whether I actually made any point.
. . . So, I hope you found that, uh. Educational? Inspirational? Entertaining?