The proper dimensions for profile pics can vary depending on which viewer you're using. You can see the proper dimensions for textures based on use at this page. It can be a bit tricky as each type of picture can have different dimensions and aspect ratios.
Aspect ratios of profile, place, etc. pictures — all of these were measured at UI size (Edit menu > Preferences > General tab > UI Size) = 1.000:
1.x Series Viewers
(official Viewer up to 1.23.5, most Third Party Viewers)
Search > All for "Classifieds", "People", and "Places" - 4:3 (256x192 pixels)
Search > Places and Classified tabs - ~7:5 (398x282 pixels)
Search > Land tab - ~7:5 (358x252 pixels)
Profile > 2nd Life tab - ~4:3 (178x133 pixels)
Profile > Picks tab - 16:9 (288x162 pixels)
Profile > 1st Life tab - 1:1 (133x133 pixels)
Profile > Classifieds tab - ~3:2 (206x137 pixels)
Profile > Web tab - 1:1 (400x400 pixels)A scrollbar uses 15 pixels on the right-hand side.
About Land > Options tab - ~3:2 (178x117 pixels)
Group Information > General tab's "Group Insignia" - 1:1 (126x126 pixels)Some of these textures are shared (for example, Search > All place pages, Search > Places, and About Land > Options use the same image), so you should pick a well-balanced ratio and size.
Learn how to get correct texture aspect ratios when editing images.
2.x Series Viewers
Search > Classifieds thumbnail - 4:3 (60x45 pixels)
Search > Classifieds expanded - ~4:3 (252x188 pixels)
Search > Classifieds expanded > Details - 17:7 (272x112 pixels)
Search > People - 4:3 (188x141 pixels)
Search > Destination Guide thumbnail - ~4:3 (45x34 pixels)
Search > Destination Guide expanded - 5:3 (250x150 pixels)
Profile > 2nd Life tab - ~1:1 (100x99 pixels)
Profile > Picks thumbnail - 8:5 (88x55 pixels)
Profile > Pick expanded - ~5:3 (285x172 pixels)
About Land > Options tab - ~20:13 (193x125 pixels)
Places > More information - ~11:7 (272x173 pixels)
Maximum texture size - 1024x1024 pixelsAll Second Life textures are constrained to powers of 2 (e.g., 128, 256, 512).
Some textures inworld have a resolution as high as 2048x2048; this is due to a previous limit that was higher.
We strongly recommend you use as small textures as possible because larger ones consume more memory and take substantially longer to load.