In my application I am both sending and receiving images through Xml. In order to facilitate this, I encode/decode the images using base64 encoding. MY encode/decode methods live in 2 objective-c categories, 1 for NSData and 1 for NSString. You call these methods on the respective data types, I have listed examples below. The methods are [NSData base64Encoding] and [NSString base64Decoding].

When sending to the server...

NSLog(@"Compressing Image: JPEG 80%");
NSData *imgData = UIImageJPEGRepresentation(scaledImage, 0.8f);    
NSString *b64String = [imgData base64Encoding];

My "b64String" variable now has a base64 encoded string representation of my image. You can now wrap this in Xml and use your normal NSURLConnection to send it to the server.

When receiving from the sever... (Make sure you are sending base64 encoded text from the server)

You will do your normal Xml parsing, here my "value" variable holds the base64 string from the server.

NSData *imgData = [value base64Decoding];
UIImage *image = [[UIImage alloc] initWithData:imgData];

You should be able to look up some common base64 encoding/decoding algorithms in Objective-C, let me know if you would like me to post my methods (they are kinda long so I obmitted them here).

Use the *.def file when you build the DLL, to specify what function names the DLL is supposed to export.

After the DLL is built, use dumpbin /exports to verify that the functions are indeed exported from the DLL.

After you have verified that the DLL is exporting functions, you can either link to them at run-time using LoadLibray/GetProcAddress, and/or you can link to them at build-time by passing the DLL's *.lib file (which was created when you built the DLL using its *.def file) as an argument to your application's linker.