I'm also fairly new to regex, but since noone else has answered I'll give it a shot.
The pipe-operator "|" is used for an OR operator.
The following REGEX should get you going somewhere.
(Match anything one or more times followed by either "JPG" or "JPEG" at the end)
Extended answer (editted after learning a bit about (e)grep):
Assuming you have a folder with following files in them:
test.jpeg, test.JpEg, test.JPEG, test.jpg, test.JPG, test.notimagefile, test.gif
(Not that creative with names...)
First we start by defining what we know about our pattern:
We know that we are looking for the end of the name. Ergo we use the "$" operand to define that each line has to end with the defined pattern.
We know that the pattern needs to be either JPEG or JPG. To this we use the pipeline "|" as an or operand.
Our pattern is now:
(Match any line ending with EITHER "JPEG" or "JPG")
However we see that in this example, the only difference is the optional "E". To this we can use the "?" operand (meaning optional).
(Mach any file ending with a pattern like: "J", followed by "P", followed by an optional "E", followed by a "G").
However we might also like to match files with lowercase letters in file name. To this we introduce the character-class "[...]". meaning match either of the following.
(Match any file ending with at pattern like: "j" or "J", followed by "p" or "P", followed by an optional "e" or "E", followed by "g" or "G")
(This could also be done using the "-i" option in grep, but I took this as an exercise in REGEX)
Finally, since we (hopefully) start to see a pattern, we can omit the unnecessary parentheses. Since there is only one optional letter ("E"), we can omit this one. Also, since the file only has this pattern to end on, we can omit the starting and ending parenthesis. Thus we simply get:
Finally; lets assume you also want to find files with ".gif"-filetype, we can add this as a second parameter:
(Here I've again added extra parenthesis for readability/grouping. Feel free to remove them if they seem obfuscating.)
Finally, I used ls and a pipeline to send all file names to (e)grep:
ls | egrep '([jJ][pP][eE]?[gG])|([gG][iI][fF])$'
Using the -i option and omitting parenthesis we can shorten it down to only:
ls | egrep -i 'jpe?g|gif$'