Adobe Illustrator is the best option because graphics are vectors, meaning you can size and resize your images without making them pixelated. You can create poster sized images or cake toppers with the same graphic and the quality of the graphic won't be affected.
Wording is important when you're writing a letter, especially if you're trying to carefully word a request. If you're looking for financial relief or forgiveness, I have new hardship letters and apology letters. If you simply want to thank someone, there are plenty of new thank you letters.