These pictures incorporate several traditional symbols of Easter. There are 5 sets of symbols: an Easter egg, peacock feather, Easter Bell, butterfly and dove. You may alternate the pictures throughout the 50 squares of the calendar or place them together by set of 10 (if you choose the latter method I'd recommend reserving the doves for last as a nice way to count down to Pentecost!). A little dab of glue stick or tape will work great for these. Here is an idea of what your calendar will look like with the pictures attached:
If you'd like to discuss these popular Easter symbols with your children in further depth, Charlotte from Waltzing Matilda has put together a great little printable booklet here. Also, Shower of Roses has some wonderful ideas on incorporating Easter symbols into your celebrations here.
I've also put together a set of 50 printable Easter prayers for use with a Family Prayer Jar that I'll be sharing early next week so be sure to come check those out if you'd like to use them along with the calendar. I'm working on sets for each liturgical season, so also come back during Ordinary Time to print the next set!