Well, do you know how hex is just a representation of binary? 0=0000,1=0001, and so on, with C=1100, E=1110, and all the other hex digits.
You basically just draw your 8*8 sprite, and then for each row make two hex digits, one for each four bits in the row. For example, 11001001 is C9. Do this and you will have 8 pairs of digits, for 16 total. Reading these left to right, top to bottom gives the hex code for the sprite.
For the reverse, put the 16 digits into 8 rows, two digits in each row. Replace each digit with the binary that corresponds to it, and you'll have an 8*8 sprite.
Maybe I forgot to mention this, but black pixels are 1's and white pixels 0's.
Hope this helps!

let's not forget http://www.ticalc.org/archives/files/fileinfo/389/38904.html

It was made specifically for this function, converting a sprite into hex code. ^ ^

Thanks guys I'm not really looking for a "Sprite-to-Hex" converter (besides to look for examples of how) because I just needed to kinda understand the basics so I can hopefully implement it into a program I plan to work on soon.

Doesn't calcgs import sprite data in a rather strange way, though?

that one gave me an hard time, though

When I tried to run it I ended up with full of missing .OCX/.DLL errors and had to go search them online to get it to work
 
To anyone wanting to use this program, watch out for viruses if you download OCX files because some sites aren't very legit

It's in the downloads sections. I still need to add a few alternatives, though.

can it import tiles from a BMP tilesheet? I mean, will it separate them in 8x8 tiles if you tell it to do so?