The word "fresco" comes from the Italian word fresco, meaning "fresh", which itself derives from a German word related to the English word "fresh".  In art, this refers to dipingere a fresco, or painting on a "fresh", still-wet coating of lime plaster, a technique used for creating durable murals.  The term entered English in the 1590s and is also seen in the related phrase al fresco, meaning "outdoors" or "in the open air".  

According to the Oxford English Dictionary (OED), the noun fresco was first used in English in 1598. The earliest citation is from a translation by physician Richard Haydocke.  It appeared in the phrase "in fresco" to describe the painting technique of working on fresh, wet plaster.  The OED also provides a later date for the first use of fresco as a verb, noting the earliest evidence is from 1849. 

According to the OED, the word "al fresco" was first used in English in the mid-1700s, with its earliest recorded instance appearing in a 1732 publication called the Whitehall Evening Post.  The term "al fresco" means "outside" or "in the fresh air" and is usually related to dining. 

Examples of "fresco" used in a sentence:  
"Thus, the art of fresco is necessarily piecemeal." 
"It has beautiful frescoes and an enormous sense of the past."
"Presumably, stucco decoration was more resistant to steam than fresco." 
"The miracle forms the subject of a celebrated fresco by Raphael in the Vatican." 
"There were no wall frescoes in here, because the walls of the most noble room in the house would have been hung with silk." 
"St Sophia's is a Unesco World Heritage Site and one of Ukraine's most significant cultural and religious monuments, with spectacular 11th Century mosaics and frescoes."

Examples of "al fresco" used in English: 
"This makes them great for using on tables when you're dining  al fresco."  
"So grab your sunglasses and dine al fresco this weekend!" 
"It became a must for tennis players and fans who dined al fresco in the shade of its trees.
"They look great when put outside and can be used to line a walkway, hung on fence posts to add a little light, or placed on tables for al fresco dining." 
"The government hopes the changes will also make it easier to open new bars in disused shops, as well as promote outdoor drinking, al fresco dining and live performances."

Probably the most famous frescoes are those in the Sistine Chapel. They are definitely not al fresco. 

Just the facts:

ALFRESCO Definition & Meaning - Merriam-Webster

7 Synonyms & Antonyms for AL FRESCO | Thesaurus.com