This post explains how to add an icon to your VSIX extension. (this was done using visual studio 2015 and relates to the following project : CExtensions-EFModelGenerator. see the Extension project)

A. Get a .ico file. You can create one or get a png file and find a converter online... there are plenty of them.

B. open your *.vsct file and add a GuidSymbol Element. To generate the guid : from file menu : Tools => Create Guid

 <GuidSymbol name="EfModelCommand_Icon_Symbol" value="{363F9640-FBB7-4403-AB20-9A53EA067ECA}">
      <IDSymbol name="EfModelCommand_Icon_ID" value="1" />
 </GuidSymbol>

To generate the guid : from file menu : Tools => Create Guid, select registry format, copy and paste it

C. Add a Bitmap Element : find the Bitmaps section and add the following

<Bitmap guid="EfModelCommand_Icon_Symbol" href="Resources\EfModelCommand_Icon.ico" 
usedList="EfModelCommand_Icon_ID"/>

D. Adapt your Button Element : Find the Button section for your command and adapt it to use your icon.

 <Button guid="guidGenerateEfModelCommandPackageCmdSet" id="GenerateEfModelCommandId" 
priority="0x0100" type="Button">
        <Parent guid="guidGenerateEfModelCommandPackageCmdSet" id="MyMenuGroup" />
        <Icon  guid="EfModelCommand_Icon_Symbol" id="EfModelCommand_Icon_ID" />
        <Strings>
          <ButtonText>EFModelgenerator Generate...</ButtonText>
        </Strings>
</Button>

E. Start a Debug session and your icon should show up.