Can you retrieve a deleted pic. on a canon sd300 digital camera!?

Yes you can. You need to get a removable media file recovery program. Be sure you download one for REMOVABLE media. There are many that only work for fixed hard drives. Do not take any more pictures on that flash card until you have recoved the image. If you have taken pictures since you deleted the image, you may be able to recover it, or it could be lost forever. Look on Download.com or tucows.com for programs that will do what you need.

You need a digital photo recovery software to do this task.

The first one is a very useful and user freindly shareware that allow you to download and install it for free. you can see the pictures that can be recovered but to recover it you must pay for the registered version.

The second one is for free, you can try it but I've never use it before.

Or if you want to search for the program yourself, just type "digital photo recovery" in your search bar.(or google)

are the deleted image on the card or on the camera's memory ?
If it's on the card, it's possible.
There are software that will be able to dump the content of the card and retrieve the jpegs. it does that find looking for the magic numbers at the beginning of a jpeg file which act like a signature.

It will work on most photos, especially if you delete all photos together when you copy them to your computer as the files will be contiguous.
examples that i have not tried http://www.cardrecovery.com/
I believe there was something similar on a cd last time i bought a sandisk memory card.