This job request appeared in Episode 2 of Fairy Tail S1
This job request appeared in Episode 30 of Fairy Tail S2
So, considering the 7 year time skip, the amount of adventures they have went on for about 200 Episodes, why has this request not been cleared yet?
Is there any significance that they re-used this particular job request again and again?
Yes, I am positive that this has appeared more than this 2 times throughout the entire Fairy Tail Anime.
As kuwaly said, it is the infamous "picture" of the Loch Ness Monster. I'm sure there is no real meaning to it, simply that it is a long-running joke in the anime. The reason they used the Loch Ness monster is likely because in real life many people visit the Scottish Highlands, specifically lake Loch Ness to try and find/prove the Loch Ness Monster. The reason they have continued to re-use the request is because, just like in real life no one has "found" the Loch Ness Monster, and because it's just a funny Easter egg that you would only notice if you payed very close attention.
Although the infamous hoax image seems to be backwards/flipped in the anime.
Similar Easter Eggs using the Loch Ness monster can be seen in these examples:
The video game Hitman: Absolution 
Hope this helps!
