Best database choose for a IM-Chat application

Hi every one i am kinda new when it comes to databases i know the very basic commands (insert delete etc.) but what i want is a database that have a high int/id limit (most likely so many that it "never" can reach a "overflow") that can hold large strings (over 10k chars would be great) that also supports a large amount of "special" chars (Chinese, Japanese, Korean and so on) images some how maybe as binary data? and datetime. at the moment am i using a SDF database that have some of the very basics stuff to my needs, but i cant have strings longer than 4000 chars and can only hold 99.999.999 entries/id's and it does not have a datetime or image entry sadly
 
i've been looking abit into MDF databases in C# but they seems to have a string limit at 4000 also as SDF otherwise it seems to suit my needs but i wanna hear in here if there is some other alternatives that could be usefull for me. also if i can avoid it i would prefer a non-stand alone database such as i belive mysql is?. also if it is possible some link to how i can connect to the database inside my C# app would be nice and if the database can be encrypted and decrypted it would be nice too.
 
so to summarize; what database should i use for a IM-chat, that can both contains large amount of entries that most likely never can/will be reached, large strings (over 10000 char is prefer) and have the most special chars from "all the world", images, and datetime parameters? also i would love if i could get some simple code, it is possible to run inside my server app and not in its own cosole/database program, and i can encrypt/decrypt selected tabels of my own choose.
 
hope to see some helpful comment soon thanks for reading and helping
 
- Jackie