Nothing wrong in such pictures. Somebody also told me that Lord Krishna with a blowing flute means that all your riches would be blown away (I was'nt worried 'cause I had no riches) but riches only increased after I kept it on my altar. (why don't you think that He is bringing melody in your life). Muruga and Venkatachalapathy always comes in standing pose only. Try not to keep 'Siva Linga' at home, because I had some bitter experience as I had one at home when I was in Delhi. (Instead you can go for a picture of Nataraja or Ardhanareeswara or Siva&Parvathi). Most astrologers say it is better to have 2 dimensional pictures instead of 3d idols at home altars.