STEP 1a: Find a public domain image.

Very easy!

Choice A: YOU CAN USE THOSE IMAGES FOR ANYTHING WITHOUT ATTRIBUTION
Go to Wikimedia Commons, type in the search tab a string plus the string "cc-zero".

Example, find dozens of images of a "temple", all public domain?
Go to http://commons.wikimedia.org/w/index.php?search=temple+cc-zero&title=Special%3ASearch

Choice B: Pick one site in this huge list: http://en.wikipedia.org/wiki/Wikipedia:Public_domain_image_resources
Many maps currently included were found into Pixabay.

STEP 1b: Find a creative commons image.
It's even easier. Always remember to ATTRIBUTE to the author the images you find!
Loads of images with CCommons license are found into the stock categories
in the site DeviantArt (c).
BUT use only CreativeCommons images MARKED for reusing (not the copyrighted ones!)

STEP 2: Make them seamless.

Seamless maps work better and are always more suitable to be used for example
in heightmapIFS. 
This step is a lot harder in general if you are a perfectionist.

I did it this way. I use Ulead Photoimpact 12 (c) and the filters;
- REDFIELD PERFECTUM (c) - Jaggy jpeg edges killer in one click.
- MEHDI FINE THRESHOLD (C) - Convert images to silhouettes + kill jaggy edges in one step!
- REDFIELD SEAMLESS WORKSHOP (c) - Helps a LOT! Normally in a few clicks you are done.
- TOPAZ ADJUST (c) - Adds blur, sharpening and lots more for dummies. Helps improving "flat images".

Another essential function from the program; CROP TOOL, you must be very precise
to crop only the repeating part of the image. Another miracle tool - PERSPECTIVE CROP TOOL,
useful to straighten and crop in one click, but not always perfect.

STEP 3a: Convert an image to a spherical map.
Useful to use a photo as a full background sphere, nicer for reflections.
Also useful for many other usages in-program... (lightmaps I suppose)

I did it this way. I use Ulead Photoimpact 12 (c) and the filter;
- FLAMING PEAR FLEXIFY 2 (c) - The settings deform the images in a lot of ways
The best setting I found is; 
INPUT = CYLINDRICAL and OUTPUT = EQUIRECTANGULAR

You get a distorted image BUT after cropping it, you can use any image as a spherical
map with a nicer distortion! All images in m3maps with number 3000+ (except a few) are
obtained with this trick.

STEP 3b: Dig in FLICKR (c)! Always remember to ATTRIBUTE to the author the images you find!

https://www.flickr.com/groups/equirectangular/pool/

It's a never ending source of stunning images that you can use as background sphere,
BUT use only CreativeCommons images MARKED for reusing (not the copyrighted ones!)