And you're sending 1,000 messages? Glad I'm not on FaceBook!
The KM manual and Wiki.
But first you need to make a plan. Do the operation manually, for one row of the spreadsheet, and note down everything you do. For example:
Make Excel spreadsheet active
Select cell A1
Copy
Open the copied URL in Safari
Make Excel spreadsheet active
Select cell B1
Copy
Switch to the frontmost page in Safari (that you opened earlier)
Click on an image
Paste
Click "Send" button
That seems long-winded (and we're about to throw half of it away!) but it makes what you want to do explicit.
If you Copy two columns in Excel then one thing on the Clipboard will be lines of tab-delimited text:
A1<tab>B1
A2<tab>B2
A3<tab>B3
...
..and KM has a "For Each" action that will loop through lines of text on the Clipboard or in a KM variable and do something to each line.
You can use "Custom Array Delimiters" to get the text before or after the tab for each line in your loop, using %Variable%Local_variableName[1]\t%
for "before" and %Variable%Local_variableName[2]\t%
for "after".
There's a "Click or Move Mouse" that can detect and click on an image, or you might be able to use the "Click Browser Link" or "Press a Button" actions. And there are "Copy" and "Paste" actions.
Putting that all together (and assuming your spreadsheet only contains those two columns):
Open the spreadsheet in Excel
Select All
Copy
Activate Safari
Set variable Local_theText to the System Clipboard
For Each line Local_line in the variable Local_theText
Open URL %Variable%Local_line[1]\t%
Click on the image
Paste %Variable%Local_line[2]\t%
Click on "Send"
End of For Each
You'll probably have to add "Pause Until" actions between various steps to stop the macro racing ahead of your web browser.
The specifics will very much depend on your setup, the browser you are using, what you actually want to do, and so on. But the above (along with the linked documentation) should be enough to get you started, so give it a go and see how you get on.