The texture doesn't need to be in the Poser directory, though it will be faster to find if you put it somewhere inside runtime/textures. The simplest way is like this: First hit Ctrl-U, or select the Material Room tab. Then use the selection pulldowns at the top of the Material room to find the object and material you want to 'paint'. In this case I've chosen the MainRoof material of the Cottage. Click in the Diffuse box, where I've written 'click here'. This will give you the file-open dialog you see off to the right. Browse until you find your JPG file, and hit OK. In most cases you'll also want to hit the 'apply to highlight' button.

My python page
My ShareCG freebies