Create a module, call it e.g. Pictures and add pictures to it. The elements.xml looks like this:
<?xml version="1.0" encoding="utf-8"?>
<Elements xmlns="http://schemas.microsoft.com/sharepoint/">
<Module Name="Pictures" Url="MyPictureLibrary">
<File Path="Pictures\pic1.png" Url="pic1.png" Type="GhostableInLibrary"/>
<File Path="Pictures\pic2.png" Url="pic2.png" Type="GhostableInLibrary" />
</Module>
</Elements>
Add the module to a web scoped feature. (Assuming you call your library MyPictureLibrary and the feature that deploys this module is activated in the same web as the library)
Edit: Or do it through code like Rob suggested.