I view radicals analogous to Latin root words (e.g. aero-, anti-). So some radicals are themselves words and pictograms. However, not all pictograms are radicals. For instance, 木 is a radical and a pictogram (looks like a tree, right?), but the words 林 and 森 all have the same root radical, 木, and are pictograms, but are not themselves radicals. A bit of trivia, when we used paper dictionaries, words were arranged by radicals.

①Is a radical just an ideogram inside another character? YES For example: "她"(she/her) "妈"(mom) "姨"(aunt) "好"(good/ok), they have common radical--女 The radicals sometimes impact on the meaning of characters, as is shown above,"她,妈,姨" are all feminine. "狗(dog)" "猫"(cat) "猪"(pig) "狐狸"(fox),etc characters with "犭" are normally animals. "病"(disease) "癌症"(cancer) "痛"(hurt) characters with "疒 " are disease or something bad. It will help you lean chinese character by heart and guess the meaning of news words the first time you mee them. ②Can every pictogram (or ideogram) work as a radical? NO. We have set radicals.

this approach doesn't seem correct to me... because the general concept is too misty for beginners, takes much time to remember what is radical, what is not and why (usually no why)... to make it simple -- Chinese symbols are pictures which consists of a set of parts 塔 = 塔 = 土 plus 艹 plus 人 plus 一 plus 口, these parts can be meaningless 艹 or meaningful 口 and some of them are chosen as index parts, to divide all symbols into certain groups