The smaller you go - the more expensive performance gets.
Acer Aspire TimelineX AS3820T 13" $530
Core i3 380M (2.53GHz) CPU 4GB RAM 500GB HDD 13.3" 1366x768 LCD w/ Intel HD Graphics.
This will do excellent on web browsing and very good at photo editing. Playing Portal on the HD graphics you'd be looking at low/medium graphics settings.
lenovo IdeaPad Z565 15" $500
AMD Turion II Dual-Core P560(2.5GHz) CPU 4GB RAM 320GB HDD 15.6" 1366x768 LCD w/ ATI Radeon HD 4270
Also excellent on web browsing and very good at photo editing. Portal on the 4270 graphics you'd be looking at medium graphics settings.
ASUS K52JE-XN1 15" $450
Pentium dual-core P6100 2.00GHz (part of the i3/i5 family) CPU 4GB RAM 320GB HDD15.6" 1366x768 LCD w/ ATI Mobility Radeon HD 5470.
Still excellent on web browsing, a notch below the other two at photo editing but still good. The HD 5470 graphics will play Portal on medium/high graphics settings.
The least expensive 13" notebook that would also play Portal on medium/high graphics settings I could find was this ASUS U31JG-A1 13" Notebook $815
Core i3 380M (2.53GHz) CPU 4GB RAM 500GB HDD 13.3" 1366x768 LCD/w NVIDIA GeForce GT 415M/Optimus + Intel HD