that remind me my childhood when I was drawing my first pcb's with asphalt dissolved in benzine ... good times, unfortunately smd technology made all those attempts seldom, the same as drawing the pcb artwork in photoshop, paint, etc. - quality is not satisfactory at all. Unfortunately sometimes I want to reproduce something that is already done, a ready design - just to give it a try, but the author provide the pcb only in .jpg format.
1. In practice I do the following - open the picture with good graphical editor, like photoshop, then make sure there are no blur areas or noise pixels between tracks, all tracks, vias, holes, etc. should be clean and clear.
2. Print the image. Use known component to calculate the re-size factor. In case that you know the dimensions of the pcb, i.e. x and y - it is easier just rescale using this.
3. Print again. Use known component or just put some components on it and check how it fits. I use smd component with many legs, like SOIC package microcontroller
4. If the component fits - this is the right size and you are ready. If it does not fit - you need to repeat the calculation and print again.
In case you are doing new design, it is really not worth to do it in photoshop, today there are free / educational licensed / trial cad/cam software and working with it is not that hard, the result will be more professional looking, easy to reproduce, always with right scale, you can copy mirrored, etc.