1. You need a JPEG-picture. This is the usual picture format in the internet. Nearly every Photo editor can convert pictures into JPEG.
2. The size of your picture should not exceed 100 KB
3. The width of your picture should not exceed 250 Pixel. Remember: Every Pixel gets converted into an ASCII-Symbol. So if a line reachs the border of the screen it breaks, and the picture can't be displayed properly. An old rule of ASCII Art says a line consits of 80 Characters.
If you follow these rules it should be easy to upload your picture to www.ascii-pics.net