It depends on how complex your needs are and what you expect from the photo app. eg. editing browsing etc.
Gimp is a good freeware substitute for Photoshop in some respects but is less intuitive than Photoshop and quite complex. Photoshop Elements (PSE) is a good low cost paid substitute for Photoshop and easier to use.
If you want a free photo app that is a great photo browser and has a solid basic level editor then I would highly recommend FASTSTONE. FASTSTONE can be used as a non installable app.
My Computer
Computer type
PC/Desktop
Computer Manufacturer/Model Number
Own build
OS
Windows 7x64 Home Premium SP1
CPU
Intel i7 2600k
Motherboard
ASUS P8Z68 Deluxe
Memory
G.Skill Ripjaws (DDR3-1600) 2x4GB
Graphics Card(s)
Nvidia GeForce GTS 450; Intel HD Graphics 3000(GT2+)